The present invention relates generally to electronic ballasts for powering discharge lamps. More particularly, the present invention relates to electronic ballasts having a self-oscillating inverter topology that is substantially load-independent in operation.
Dimming and program-start lamp ballasts are well known in the art for their advantageous lighting features. Electronic ballasts are frequently provided for their superior ability to provide such features. Unfortunately, their complexity and resulting cost can be prohibitive for certain applications, as one or more dedicated control circuits such as a microcontroller and/or switch driver integrated circuit are required to provide the desired switching operations. This is particularly the case where more than one lamp is provided for a common light fixture.
Self-oscillating inverter topologies as are known in the art provide for less complexity and therefore lower cost in ballast design, but such topologies tend to be dependent on load condition (i.e., light load, heavy load and most particularly no load) such that the inverter operating frequency changes with variations in the load condition. This can make it difficult to maintain a desired frequency. Without a microprocessor to detect fault conditions and shut down the ballast, it is also desirable if not necessary to provide protection circuitry for circuit components in the event of open load and short load fault conditions. Further, such topologies as are currently known in the art generally include capacitive mode switching (i.e., hard-switching) during transient operation, where inductive mode switching (i.e., soft-switching) is much more desirable to reduce stresses on circuit components.